How Does the Petroleum Densimeter Method Measure Density? Engineering Application of Archimedes' Principle

The HZMD-2001 utilizes the densitometer method, with the instrument providing a precise constant temperature environment, and the actual measurement of density being carried out using a petroleum densitometer. Many people have been using constant temperature bath densitometers for a long time, but they cannot explain how a petroleum densitometer measures density. Clarifying this principle will naturally help understand the many detailed requirements during operation.

Archimedes' principle of buoyancy

An oil densitometer is a sealed glass tube, with a weighted ball filled with lead or steel shot at the bottom (to enable it to float vertically) and a slender graduated rod at the top. When the densitometer is placed in a liquid, according to Archimedes' principle, the buoyancy generated by the volume immersed in the liquid equals the gravitational force of the densitometer itself.

The gravitational force of the densitometer is fixed (with a known mass), and the buoyant force equals the product of the liquid density, the displaced volume, and the acceleration of gravity. For liquids with higher densities, a smaller displaced volume is required to generate sufficient buoyant force, causing the densitometer to sink shallower, with more of the scale rod protruding above the liquid surface, and a smaller reading scale. For liquids with lower densities, the densitometer sinks deeper, resulting in a larger reading.

Therefore, the numerical value on the scale rod directly corresponds to the liquid density - the scale of the densitometer has been precisely calibrated at the factory using liquids of known density.

Reading specification

When reading the densitometer value, there is a detail prone to error: the position of the line of sight and the handling of the meniscus.

Transparent liquid: When observing, keep your eye level with the upper edge of the meniscus (i.e., look straight ahead), and read the value where the upper edge of the meniscus is tangent to the scale of the densitometer. The upper edge of the meniscus is the highest point formed by the liquid surface bending due to surface tension around the densitometer rod tube, rather than the lowest point.

Dark-colored liquids (such as heavy lubricating oil and crude oil): Due to the dark color of the liquid, it is not possible to see through it. Therefore, the observer should position their eye slightly above the liquid surface and read the point where the upper edge of the meniscus is tangent to the scale of the densitometer.

Why is temperature control crucial

The density of a liquid changes significantly with temperature, decreasing as the temperature rises (thermal expansion of liquid) and increasing as the temperature drops. The density-temperature coefficient of petroleum products is approximately 0.0006 to 0.0009 g/cm³/℃, meaning that a 1℃ change in temperature results in a density change of approximately 0.0006 to 0.0009 g/cm³.

If the measured temperature differs by 5℃ from the standard temperature (usually 20℃), the density error will reach 0.003 to 0.005 g/cm³, which is completely unacceptable for petroleum trade measurement. This is why a densitometer requires a constant temperature bath. The temperature control accuracy of ±0.1℃ of the HZMD-2001 ensures the reliability of measurement data.